Optical spectrum analyzers are used for analyzing the output light beams from lasers, light emitting diodes (LED's) and other light sources. Optical spectrum analyzers are particularly useful for analyzing light sources for optical telecommunication, where it is preferable to insure that the optical carrier includes only a single, spectrally pure wavelength. In optical spectrum analyzers, the light intensity is displayed as a function of wavelength over a predetermined wavelength range, as in spectrum analyzers for lower frequency applications. Parameters of importance in optical spectrum analyzers include wavelength range, wavelength and amplitude accuracy, resolution, measurement speed, polarization dependence, sensitivity and dynamic range. As used herein, dynamic range refers to a "close-in" dynamic range and is a measure of the ability of the instrument to measure a low amplitude optical signal that may be separated in wavelength by 0.5-1.0 nanometer from a large amplitude signal. This parameter is of particular importance in analyzing the spectral purity of lasers, such as DFB lasers.
Prior art techniques for optical spectrum analysis have included the use of Michelson interferometers, Fabry-Perot interferometers and monochromators which use a prism or a diffraction grating for spatial dispersion of the input light beam. Michelson interferometers are characterized by poor dynamic range and slow operation. Fabry-Perot interferometers have high resolution but have problems which arise from the multimode nature of the optical cavity.
In a monochromator having a diffraction grating, an input light beam to be analyzed is collimated and is directed at the diffraction grating. The light beam is spatially dispersed by the grating, since different wavelengths are diffracted at different angles. The grating is rotated so that the dispersed light beam is scanned over a slit. The light that passes through the slit is detected to provide an output signal that represents amplitude as a function of wavelength. The width of the slit, the input image size, the f number of the system and the dispersion of the grating establish the resolution of the monochromator.
Single stage monochromators with diffraction gratings have been used in optical spectrum analyzers. Although a single stage monochromator has relatively low cost and high sensitivity, it has a relatively small dynamic range. In addition, when the single stage monochromator has an output optical fiber, the aperture of the output optical fiber limits the optical bandwidth that can be observed. Another problem inherent in single stage monochromators is that the output optical detector must have an effective aperture as large as the widest resolution bandwidth. The larger detector increases the stray light power and the amount of noise at the detector output. Finally, the inherent time dispersion that occurs in a single stage monochromator limits the bandwidth of the modulation that can be observed.
Two stage monochromators with diffraction gratings have eliminated some of the disadvantages of single stage monochromators, but have created other problems. A two stage monochromator involves a second monochromator in series with the first monochromator. Although the two stage monochromator provides relatively high dynamic range, there is a 10-15 dB loss in sensitivity as compared with a single stage monochromator. In addition, accurate synchronization between the diffraction gratings in the first and second stages is difficult, and optical coupling between stages can be difficult. For example, when an optical fiber is used for coupling between stages, the resolution bandwidth is limited by the optical fiber. Furthermore, the two stage monochromator is relatively complex and expensive.
The efficiency of a diffraction grating is dependent on the polarization of the incident light. As a result, the amplitude of the diffracted light beam may vary for input light beams of constant amplitude but different polarizations. Thus, optical spectrum analyzers require a compensation technique to reduce or eliminate polarization sensitivity. Prior art compensation techniques have included polarization scrambling and also spatial separation of the input light beam into two polarizations which are separately analyzed, with different amplifier gains applied to each polarization. In one prior art technique, the input light beam is separated into two spatially separate linear polarizations oriented at 45.degree. relative to the s and p polarizations as defined by the grating. This gives an average efficiency for both beams. All of the known prior art polarization compensation techniques have had one or more disadvantages, including added complexity and difficulty in coupling the light beam to an output optical fiber.
Another problem associated with scanning monochromators is the difficulty in coupling the output light beam to an optical fiber. In particular, mechanical tolerances associated with scanning of the diffraction grating cause the output beam to wander as the grating is rotated. Thus, the output beam position changes as a function of wavelength. In one prior art system, a manual mechanical adjustment is required when the selected wavelength range is changed. In another prior art system, the user must initiate a mechanical adjustment routine when the wavelength range is changed. In either case, alignment is required whenever the wavelength parameters of the measurement are changed.
It is a general object of the present invention to provide improved optical spectrum analyzers.
It is another object of the present invention to provide improved monochromators.
It is a further object of the present invention to provide a double-pass scanning monochromator that utilizes a single diffraction grating.
It is yet another object of the present invention to provide a scanning monochromator that is relatively insensitive to the polarization of the input light beam.
It is a further object of the present invention to provide a scanning monochromator wherein an output optical fiber automatically tracks the output light beam during rotation of the diffraction grating.
It is yet another object of the present invention to provide a scanning monochromator having a large dynamic range and high sensitivity.
It is yet another object of the present invention to provide an optical spectrum analyzer that is easy to manufacture and is low in cost.